import turtle

pos = [340, 0]
def draw():
    turtle.clear()
    
    turtle.up()
    turtle.goto(pos)
    turtle.down()
    turtle.dot(30)
    pos[0]-=20
    if pos[0] > -320:
        turtle.ontimer(draw, 300)

turtle.tracer(False)
draw()
turtle.done()

